Voltage regulators are used to provide a stable voltage source to other electronic circuits. Low voltage drop out (LDO) regulators are widely used in modern applications since the operation voltages of the modern electronic devices are going lower and lower than an external supply voltage. FIG. 1 schematically and generally illustrates an LDO regulator 100 of prior art. A battery voltage (i.e. external supply voltage) VBAT, which is 4.3V, for example, is supplied to the LDO regulator 100 as an input voltage. The LDO regulator 100 comprises multiple sub-LDO regulators 110, 120, . . . , 190. Each sub-LDO regulator is used to provide a specific output voltage (e.g. VOUT1, VOUT2 . . . or VOUTN). Taking the sub-LDO regulator 110 as an example, the sub-LDO regulator 110 has a control stage 112, an output stage 114 and a compensation block 113 connected between the control stage 112 and the output stage 114. The external supply voltage VBAT is supplied to the control stage 112 and the output stage 114. This is similar to the other sub-LDO regulators. Since the entire LDO regulator 100 sustains the high voltage, elements (e.g. transistors) of great sizes must be used. Alternatively, a cascade structure must be utilized. To save a layout area for the LDO regulator, a pre-regulator is added as shown in FIG. 2.
FIG. 2 schematically and generally illustrates another LDO regulator 200 of prior art. The like reference numbers in FIG. 1 and FIG. 2 indicate the same components. The difference between the LDO regulators 100, 200 of FIG. 1 and FIG. 2 is that the LDO regulator 200 further has a high voltage (HV) regulator 205. The HV regulator 205 converts the high input voltage VBAT (e.g. 4.3V) to a lower voltage such as 2.8V or 3.3V. The lower voltage from the HV regulator 205 is then provided to a control stage 212 of a sub-LDO regulator 210. The battery voltage VBAT is still fed to an output stage 214. This is similar to the other sub-LDO regulators 220 to 290.
The battery voltage (i.e. the external supply voltage) VBAT usually includes an AC perturbation having a peak-to-peak value of about 200 mV in addition to a DC component of 4.3V in this example. After the battery voltage VBAT passes through the HV regulator 205 and is converted into a converted voltage VCON, the DC component is converted from 4.3V to 2.8 or 3.3V, for example. Furthermore, the AC perturbation is filtered out. The electrical signal at a node A (i.e. VBAT) in FIG. 2 includes the DC component and the AC perturbation, while the electrical signal at a node B (i.e. VCON) only has the converted DC voltage. Therefore, the effect of the AC perturbation cannot be suppressed, resulting in degradation of a Power Supply Rejection Ratio (PSRR) characteristic of the LDO regulator 200. In addition, the use of the HV regulator 205 requires an additional power consumption and an additional occupation of the layout area.